1. Field of the Invention
This invention relates to a method of driving a droplet jetting head that jets droplets from orifices. More particularly, this invention relates to a method of driving a droplet jetting head that can suppress curvature of the tail of a droplet jetted from a nozzle orifice and improve the accuracy of landing of a droplet.
2. Description of Related Art
A droplet jetting head like an inkjet print head that jets droplets from nozzle orifices to record images with micro ink droplets jets a droplet by generating a pressure in a pressure chamber to land in a recording medium such as recording paper and the like.
There have been various devices to give a pressure to a pressure chamber. The droplet jetting head to be explained here has a pressure chamber surrounded with walls of piezoelectric element and jets an ink droplet through a nozzle orifice by deforming the piezoelectric element. The droplet jetting head is briefly explained below with reference to FIG. 1 to FIG. 4.
FIG. 1 shows a shear mode type ink-jet print head (simply abbreviated as a print head in the description below) which is an embodiment of the droplet jetting head. In details, FIG. 1(a) is a perspective view of the print head with a partial sectional view. FIG. 1(b) is a sectional view of the print head having an ink feeder. FIG. 2 shows how the print head works. FIG. 3 shows jetting of a droplet. FIG. 4 shows a waveform to drive a print head.
Referring to FIG. 1, the print head consists of an ink tube 1, a nozzle member 2, nozzle orifices 3, a partition wall S, a cover plate 6, ink inlets 7, and a substrate 8. Referring to FIG. 2, an ink chamber is formed by the partition wall S, the cover plate 6, and the substrate 8.
Although FIG. 1(b) shows the sectional view of one ink channel A having one nozzle orifice 3, the actual shear mode print head H has a plurality of ink channels A1, A2, . . . , An isolated from each other by partition walls S1, S2, . . . , Sn+1 between the cover plate 6 and the substrate 8. One end of each ink channel (sometimes called a nozzle end) is communicated with a nozzle 3 which is formed on the nozzle member 2. The other end of each ink channel (sometimes called a manifold end) is connected to an ink tank (which is not shown in the figure) via an ink inlet 7 that forms the ink feeder and an ink tube 1. The nozzle 3 forms an ink meniscus.
Each partition wall (S1, S2, . . . ) consists of a partition wall Sa (S1a, S2a, . . . ) and Sb (S1b, S2b, . . . ) which have different polarization directions as shown by arrows in FIG. 2. The partition wall S has electrodes Q1 and Q2 in close contact with the wall S1 and the partition wall S2 has electrodes Q3 and Q4 in close contact with the wall S2. Similarly, each partition wall has electrodes in close contact with the wall and the electrodes (Q1, Q2, . . . ) are electrically connected to a driving pulse generating circuit.
In the status of FIG. 2(a), electrodes Q1 and Q4, for example, of the print head H are grounded and driving pulses made of square waves of FIG. 4 are applied to electrodes Q2 and Q3. At the first rise (P1) of the driving pulse, an electric field generates perpendicularly to the polarization direction of the piezoelectric material that constitutes the partition walls S1 and S2. This electric field causes a shear deformation on the junction of partition walls S1a and S1b. Similarly, an opposite shear deformation generates on the junction of partition walls S2a and S2b. Consequently, the partition walls S1 (S1a and S1b) and S2 (S2a and S2b) respectively move outwards and increase the volume of the ink channel A1. This volume expansion generates a negative pressure in the ink channel A1 and causes ink to be sucked into the ink channel A1. At the same time the pressure in the ink channel starts to increase at both the manifold and nozzle ends and the acoustic pressure wave is propagated toward the center of the ink channel. Then the acoustic pressure wave reaches the opposite end and consequently the ink channel has a positive pressure.
When the potential of the pulse is dropped down to 0 (P2) a preset time later after the first driving pulse was applied, the partition walls S1 and S2 return to their neutral positions of FIG. 2(a). As the result, a high pressure is applied to the ink in the ink chamber.
Then, a driving pulse (P3) is applied to deform the partition walls S1 (S1a and S1b) and S2 (S2a and S2b) in the opposite direction as shown in FIG. 2(c) and reduce the volume of the ink channel A1. This generates a positive pressure in the ink channel A1. This positive pressure causes the ink meniscus (part of the ink in the ink channel A1) to change to be pushed out through the nozzle orifice. An ink pillar protrudes from the nozzle orifice. (See FIG. 3(a).)
This state is kept for a preset time period and the potential of the pulse is dropped down to 0 (P4). The partition walls S1 and S2 return to their neutral positions from the retracted positions. This increases the volume of the ink channel A1 and draws in the ink meniscus. At the same time, the rear end of the protruded ink pillar is pulled back. As the result, the ink pillar 100 separates from the meniscus and flies as a droplet 101. (See FIG. 3(b).)
As explained above, the print head H is characterized by applying positive and negative pressures to the ink in the ink channel by deformation of the partition wall S, wherein the partition wall S constitutes a pressurizing device.
In general, a droplet just jetted from a nozzle orifice consists of a main droplet body 101a which is approximately ball-shaped as shown in FIG. 3(b) and a tail 101b which extends long from the rear end of the main droplet body 101a. As the droplet flies, the tail 101b breaks into smaller secondary droplets 101c called satellite droplets. This ball-shaped main droplet body 101a and the secondary droplets 101c (satellite droplets) fly together toward a recording medium 200. When they (101a and 101c) hit the medium 200, an image part is recorded on the medium. When they (101a and 101c) fly in the same direction, they land in the same point and do not deteriorate the image quality. However, if the secondary droplets 101c fly away from the main droplet body 101a, they 101c land near the touchdown site of the main body droplet as shown in FIG. 3(b). This blurs the image part.
The reason why the secondary droplets 101c fly away from the main droplet body 101a is that the tail 101b of a droplet 101 just jetted from a nozzle orifice 3 has a curve that goes away from the flying direction (shown by an arrow in FIG. 3(b)) of the main droplet body.
Conventionally, various technologies been disclosed to improve image deterioration due to curves of droplet tails. For example, Patent Documents 1 and 2 disclose technologies by reducing the volume of a pressure chamber to increase the pressure in the pressure chamber, protruding an ink pillar from a nozzle orifice, keeping this state for a preset short time, rapidly removing the deformation of the pressure chamber, and thus shortening the tail of the droplet by this rapid expansion of the pressure chamber. This technology quickens separation of a droplet and makes the short droplet tail fly in the same flying direction of the main droplet body.
Patent Document 3 discloses a technology to prevent the droplet tail from bending by giving the first pulse to protrude an ink pillar from a nozzle orifice, giving the second pulse before the droplet separates from the nozzle orifice to protrude an ink meniscus from the nozzle orifice and separating the droplet at the top of the bulging meniscus.
Patent Document 1: Japanese Non-examined Patent Publication Hei 04-290748
Patent Document 2: Japanese Patent Publication 2693656
Patent Document 3: Japanese Non-examined Patent Publication Hei 02-215537
It has been well known that the curving of a tail of a droplet jetted from a nozzle orifice is caused by unevenness of the inner wall of the nozzle orifice. For example, when the inner wall of the nozzle orifice is slanted unevenly or partially irregular, the surface tension of the ink meniscus inside the nozzle orifice becomes unbalanced as shown in FIG. 5, a force perpendicular to the flying direction of the droplet acts on the droplet tail. This causes the tail to curve just after the droplet detaches from the meniscus M. Therefore, the degree of evenness in the shape of the inner surface of the nozzle orifice greatly has an influence on the stable flight of a droplet without a curve on its tail.
The technologies disclosed by Patent Documents 1 and 2 suppress the influence by the shape of the internal wall of a nozzle orifice by shortening the tail of a droplet jetted from a nozzle orifice and thus quickly separating the droplet from the meniscus. These technologies separate the droplet from the meniscus earlier to shorten the length of the droplet tail and specifically separate a droplet before the meniscus returns to the nozzle orifice. Therefore, it takes a long time for the next droplet to be ready for jetting and a driving frequency may drop. Further, the droplet jetting heads have been used in various fields and forced to use liquids of various properties. Some kinds of liquid cannot be free from having longer droplet tails. As explained above, long droplet tails are easily affected and curved by the forms of inner walls of the nozzles.
To suppress curving of the droplet tail, the inner surface of a nozzle must preferably be a perfect circle in cross section and symmetrical relative to the center of the nozzle orifice. However, it requires a very high working precision when forming a perfect and symmetrical circle in the inner surface of the nozzle and this is very hard. So it is impossible to meet the requirement.
Further, if an unwanted object adheres to the inner surface of the nozzle in use, it is hard to be removed. This object may cause the droplet tail to curve.
So the other ways have been demanded to jet droplets steadily without tail curving instead of making the nozzle inner circles as perfect as possible. As described above, the technology disclosed in Patent Document 3 separates a droplet after protruding a meniscus from the nozzle orifice. This technology can suppress the influence due to the condition of the inner nozzle wall, but uses a second pulse to bulge a liquid meniscus in addition to the first pulse to protrude an ink pillar. So this technology must cancel vibrations caused by this second pulse, but this reduces the driving frequency.
Judging from the above, an object of this invention is to provide a method of driving a droplet jetting head that can steadily jet droplets without droplet tail curves, wherein the tail shapes are not affected by the influence due to the condition of inner nozzle surfaces and the driving frequency is not reduced.
Other objects of this invention will be apparent from the description below.